1. Field of the Invention
The present invention relates to a lithium ion secondary battery, and more specifically, to a lithium ion secondary battery having a polygonal section, which can enhance safety and reliability thereof by allowing a safety vent to be easily destroyed under a low pressure.
2. Description of the Related Art
In general, a lithium ion secondary battery having a diagonal section comprises an electrode assembly formed by a positive electrode plate, which is wound into a jelly roll shape and to which a positive electrode active material is attached, a negative electrode plate to which a negative electrode active material is attached, and an isolation film which is positioned between the positive electrode plate and the negative electrode plate, to prevent a short-circuit and to enable only movement of lithium ions; a can to which the electrode assembly is coupled; a cap assembly which closes the can and prevents the electrode assembly from being detached from the can; and an electrolyte which is injected into the can and allows lithium ions to be moved in the can.
In order to form the lithium ion secondary battery, first, the positive electrode plate to which the positive electrode active material is attached, the negative electrode plate to which the negative electrode active material is attached and the isolation film are stacked. This stacked structure is wound up in a jelly roll shape, and is put into the can having a diagonal section through an opening of the can. Thereafter, the cap assembly is welded to the opening of the can. Then, the electrolyte is injected into the can through an inlet of the cap assembly, and then the inlet is closely sealed. A bare cell is completed through a formation process of carrying out charge and discharge several times. Subsequently, a protection circuit is attached to the bare cell, and then assembly and inspection processes are carried out, thereby completing a conventional battery pack.
On the other hand, since the lithium ion secondary battery employs a constant potential current regulated charge method, an over-charge phenomenon is not generated only if a charge voltage of a charger is accurately controlled. However, an abnormal charge may occur due to destruction or erroneous operation of the charger. In this case, since the positive electrode active material such as lithium cobalt oxide (LiCoO2) has a characteristic that the potential is continuously increased, a cell voltage is continuously increased, and thus an abnormal heat emission phenomenon occurs.
Safety measures for such phenomenon include a measure of having a PTC (Positive Temperature Coefficient) element built-in, a measure of adopting an isolation film having a shut-down operation, etc. In addition, as the safety measure, there is a safety vent working by means of generation of gases. Here, the safety vent of the lithium ion secondary battery having a diagonal section indicates a portion usually formed with a thin thickness by forming a notch on a bottom surface of the can or an outer surface of the cap assembly, and the safety vent is destroyed in a case where a swelling phenomenon is severely advanced due to generation of gases, thereby venting the gases.
The gases are generated because the lithium carbonate Li2CO3 that is used for the formation of the positive electrode active material such as lithium cobalt oxide LiCoO2 is added excessively over the Stoichiometry. The excessively added lithium carbonate remains in the lithium cobalt oxide LiCoO2 as the positive electrode active material in a non-reaction state. Then, the lithium carbonate Li2CO3 is decomposed when a voltage of the battery is increased and heat is generated due to the abnormal charge, thereby generating carbon oxide gas. The swelling phenomenon where the can is excessively expanded results from generation of the carbon oxide gas, and when the swelling phenomenon is severely advanced, the safety vent operates.
The swelling phenomenon may be solved by adding a small amount of lithium carbonate, but in this case, the cobalt oxide CoO2 remains in the positive electrode active material. Cobalt is eluted into the electrolyte during charging due to the remaining cobalt oxide, and the eluted cobalt agglomerates at the surface of the negative electrode, thereby causing an internal short-circuit, which is more dangerous. Therefore, the lithium carbonate should be excessively added in preparing the positive electrode active material.
Due to the swelling phenomenon, a wide plane of the can is expanded outwardly, and the other four narrow planes are bent outwardly at both ends thereof and are contracted inwardly at the central portions thereof.
Conventionally, the safety vent is formed by forming a notch on a wide plane or on a bottom plane of the can or on an outer surface of the cap assembly by using a pressing method, and when the swelling phenomenon occurs, the safety vent operates.
However, if the notch is formed on the outer surface of the wide plane of the can, where a large tensile stress is applied, the notch lessens the tensile stress imposed at the part when the wide plane expands outwardly.
As a result, from the viewpoint of rigid body mechanics, the safety vent cannot be destroyed properly with a desired pressure but may be destroyed with a very high pressure. Thus, there is a problem that safety of the battery is much deteriorated.
Therefore, a pressing method of forming the notch of the safety vent in a large depth has been developed, but this method has a problem that the safety vent is destroyed with various pressures. As a result, this safety vent provides poor reproducibility and thus the reliability is deteriorated.